Debug help - Programming in D - extending tail slice terminates sharing unexpectedly. Page 69, 70

[Serg Gini]

On Monday, 20 October 2025 at 09:50:35 UTC, Brother Bill wrote:
> So all that guidance of tail appending using 'length' and 
> 'capacity' is obsolete.
> That is, if the capacity > length, then one can safely extend 
> the tail by (capacity - length) elements.
> The new advice is to just not append to slices either for the 
> base array or any tail slice array.  Otherwise, breakage or 
> termination of slice sharing may result.
>
> Do I have that correct?

I've played a bit with code:
```d
import std.stdio;

void main()
{
     int[] slice = [1, 3, 5, 7, 9, 11, 13, 15];
     int[] tailSlice = slice[$ / 2 .. $];
     writeln("slice: ", slice, " length: ", slice.length, " 
capacity: ", slice.capacity, "address: ", slice.ptr);
     writeln("tailSlice: ", tailSlice, " length: ", 
tailSlice.length, " capacity: ", tailSlice.capacity, " address: 
", tailSlice.ptr);
     writeln("---");
     slice[6] = 99;
     tailSlice[1] = 55;
     writeln("slice: ", slice, " length: ", slice.length, " 
capacity: ", slice.capacity, "address: ", slice.ptr);
     writeln("tailSlice: ", tailSlice, " length: ", 
tailSlice.length, " capacity: ", tailSlice.capacity, " address: 
", tailSlice.ptr);
     writeln("---");
     slice ~= 100;
     writeln("slice: ", slice, " length: ", slice.length, " 
capacity: ", slice.capacity, "address: ", slice.ptr);
     writeln("tailSlice: ", tailSlice, " length: ", 
tailSlice.length, " capacity: ", tailSlice.capacity, " address: 
", tailSlice.ptr);
     writeln("---");
     slice[7] = 66;
     tailSlice[0] = 44;
     slice ~= 111;
     writeln("slice: ", slice, " length: ", slice.length, " 
capacity: ", slice.capacity, "address: ", slice.ptr);
     writeln("tailSlice: ", tailSlice, " length: ", 
tailSlice.length, " capacity: ", tailSlice.capacity, " address: 
", tailSlice.ptr);
}
```

```
slice: [1, 3, 5, 7, 9, 11, 13, 15] length: 8 capacity: 11address: 
7FAE68344000
tailSlice: [9, 11, 13, 15] length: 4 capacity: 7 address: 
7FAE68344010
---
slice: [1, 3, 5, 7, 9, 55, 99, 15] length: 8 capacity: 11address: 
7FAE68344000
tailSlice: [9, 55, 99, 15] length: 4 capacity: 7 address: 
7FAE68344010
---
slice: [1, 3, 5, 7, 9, 55, 99, 15, 100] length: 9 capacity: 
11address: 7FAE68344000
tailSlice: [9, 55, 99, 15] length: 4 capacity: 0 address: 
7FAE68344010
---
slice: [1, 3, 5, 7, 44, 55, 99, 66, 100, 111] length: 10 
capacity: 11address: 7FAE68344000
tailSlice: [44, 55, 99, 66] length: 4 capacity: 0 address: 
7FAE68344010
```

But I'm not sure about slice capacity. it doesn't make any sense 
for me when it is non-zero
https://dlang.org/spec/arrays.html#capacity-reserve